Add Manpage artifact

Library/Homebrew/cask/artifact.rb

Library/Homebrew/cask/artifact/manpage.rb

Library/Homebrew/cask/config.rb

Library/Homebrew/cask/dsl.rb

Library/Homebrew/rubocops/cask/constants/stanza.rb

Library/Homebrew/test/cask/artifact/manpage_spec.rb

...ry/Homebrew/test/support/fixtures/cask/AppWithManpage.zip

...fixtures/cask/Casks/invalid/invalid-manpage-no-section.rb

.../fixtures/cask/Casks/with-autodetected-manpage-section.rb

...port/fixtures/cask/Casks/with-explicit-manpage-section.rb